Method of producing composite tubes



Aug. 23, 1966 R. E. SEATON 3,267,553

mmoo 0F rnonucme COMPOSITE TUBES Filed Sept. 50, 1963 INVENTOR. RALPij E. SEATON ATTORN EYS United States Patent 3,267,563 METHOD OF PRODUCING COMPOSITE TUBES Ralph E. Seaton, Detroit, Mich., assignor to Calumet & Hecla, Inc, Allen Park, Mich, a corporation of Michigan Filed Sept. 30, 1963, Ser. No. 312,453 8 Claims. (Cl. 29-1573) The present invention relates to a method of producing a tubular device comprising an internally finned inner or liner tube, and a smooth outer or shroud tube in radial pressure contact with the inner or liner tube.

It is an object of the present invention to provide a method of producing a composite tubular device comprising an internally finned liner tube formed of a first metal, and a smooth shroud tube having its inner surface in continuous radially acting pressure contact with the outer surface of the liner tube.

It is a further object of the present invention to provide a method of producing a tubular device as described in the preceding paragraph in which the inner tube is formed of aluminum.

It is a further object of the present invention to provide a method of producing a tubular device as described in the preceding paragraphs in which the shroud tube is formed of copper.

It is a further object of the present invention to provide a novel method of producing a composite tubular device which comprises extruding a liner tube provided with internal longitudinally extending fins and a smooth outer surface, assembling the liner tube in an outer shroud tube having an internal diameter slightly greater than the outside diameter of the liner tube, the shroud tube having smooth inner and outer surfaces and having a yield strength substantially less than the yield strength of the liner tube, reducing the diameter of the shroud tube sufficiently to materially stress the liner tube so as to produce continuous radially acting pressure contact between the outer surface of the liner tube and the inner surface of the shroud tube.

It is a further object of the present invention to provide a method as described in the preceding paragraph in which the outer shroud tube is sunk onto the inner liner tube by drawing the assembly through a die.

It is a further object of the present invention to provide a method as described in the preceding paragraphs in which the extruded inner liner tube is aluminum and the outer shroud tube is copper.

Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings, illustrating a preferred embodiment of the invention, wherein:

FIGURE 1 is an end view of the composite tubular device.

FIGURE 2 is an end view of the inner liner tube prior to assembly.

FIGURE 3 is an enlarged fragmentary sectional view of the inner liner tube.

The tubular device indicated generally at 10 in FIG- URE 1 comprises an inner tube 12 provided with a multiplicity of generally radially inwardly extending fins 14 which are tapered as best illustrated in FIGURE 3.

In a tubular device of the character described, which is used for a water chiller tube, excellent results have been obtained when the fins have a height of approximately .100", a thickness at the base of approximately .035", and a thickness at the tip of approximately .010, the fins being formed to extend inwardly from a tube having a wall thickness of approximately .025 and a diameter of approximately 0.65. A tube having these dimensions is readily extruded from a suitable aluminum Patented August 23, 1966 alloy intended for extrusion, and excellent results are obtained using 6063-T5 aluminum. The aluminum liner tube after extrusion undergoes an age hardening treatment for a purpose which will present appear.

In use as a water chiller tube the environment requires that the outer surface of the tubular device be of a metal other than aluminum and copper has been found to be entirely satisfactory for this purpose, due in part to its resistance to corrosion.

Inasmuch as the tubular device is intended to operate as a heat transfer device, it is of course apparent that heat transfer across the outer surface of the liner tube and the inner surface of the shroud tube shall be as high as possible. In order to accomplish this it is essential that these surfaces be in smooth continuous contact which can best be assured by maintaining the surfaces under continuously acting radial pressure.

To produce the tubular device the inner tube 12 is inser-ted into an outer or shroud tube 16 formed of a corrosion resistant material, preferably copper. Excellent results are obtained when the shroud tube is formed of DHP copper. The copper tube is formed with an internal diameter sufficiently greater than the external diameter of the extruded aluminum tube so as to permit ready assembly. Excellent results are obtained when the aluminum liner tube is extruded to have an outside diameter of 0.680/ 0.690. The copper shroud tube, which is initially formed to fit loosely over the inner liner tube is produced with an outside diameter of 0.790/ 0.800", and with a wall thickness of 0.036".

In order to insure the permanent result of radially acting pressure between the inner surface of the shroud tube and the outer surface of the liner tube, the yield strength of the inner tube must be substantially greater than that of the shroud tube. With the materials specified, the minimum yield strength of the copper shroud tube is approximately 6,000 p.s.i., and the yield strength of the aluminum liner tube, after the age hardening thereof, is approximately 16,000 p.s.i.

After loosely assembling the aluminum liner in the copper tube, the assembly is pulled through a die having an aperture of 0.750". This operation creates an intimate pressure contact between the outside diameter of the liner and the inside diameter of the copper shroud tube which provides a low resistance thermal conductive path.

The effectiveness of the continuous pressure contact between the adjacent surfaces of the tube is attributed to the difference in yield strength of the component parts. In tube drawing the material tends to spring back to a diameter somewhat larger than the diameter of the die through which it is pulled. This spring back effect is greater in materials having greater yield strength. It follows therefore, that when the two components of this assembly are drawn through a die, with the component having the greater yield strength on the inside, the inside component will spring back more than the outer component. This differential spring back will create a permanent stress condition at the interface and provide a bond or contact between the two surfaces characterized by a low resistance thermal conductive path.

In some cases it is necessary or desirable to sink or reduce the diameter of the extruded aluminum liner tube after extrusion and prior to the assembly in final draw operation with the copper shroud tube. In such case, the aluminum liner tube is aged subsequent to sinking and before assembly with the copper shroud tube.

As a matter of interest, it is noted that in the specific example described in the foregoing, the copper tube is subjected to an 8% reduction in cross-sectional area by the draw operation.

The composite tubular device described in the foregoing is particularly useful as a water chiller tube because the inner surface of the liner is provided with fins which produce a total inside area of contact approximately twice as great as the outside area. Since the outside of the tube is in contact with water and the inner surface of the tubular device is in contact with a chilled gaseous medium, this area ratio produces a highly eiiicient overall heat transfer relationship. The eflicient tubular device permits the formation of the internally finned'structure by highly eflicient extrusion of a material designed for this purpose, while at the same time, it is characterized by the external corrosion resisting surface of a dissimilar metal, specifically copper, to be assembled together by an operation which insures a permanent radially acting pressure between the smooth adjacent surfaces of the shroud and liner, thus producing a thermal conductive path of low resistance. 7

The drawing and the foregoing specification constitute a description of the improved tubular device in such full, clear, concise and exact terms as to enable any person skilled in the art to practice the invention, the scope of which is indicated by the appended claims.

What I claim as my invention is:

1. The method of making a composite tubular device which comprises eftruding a liner tube having a smooth outer surface and 'a'finned inner surface, assembling over said liner tube a shroud tubehaving smooth inner and outer surfaces and having a yield strength substantially less than the yield strength of the liner tube, sinking the shroud tube onto the liner tube by drawing the assembly through a die sized to materially stress the liner tube.

2. The method as defined in claim 1 in which the liner tube is formed of aluminum.

3. The method as defined in claim 1 in which the shroud tube is formed of copper.

4. The method as defined in claim 1 in which the liner tube is formed of 6063-T5 aluminum.

5. The method as defined in claim 1 in which the shroud tube is formed of DHP copper.

6. The method as definedin claim 1 in which the liner tube is formed of aluminum and in which the shroud tube is formed of copper.

7. The method of making a composite tubular device which comprises extruding an internally finned aluminum tube having a smooth outer surface, positioning an outer copper tube having a smooth inner surface over'said ex truded aluminum tube, and thereafter drawing said tubes to produce a permanent reduction in the outside diameter of said inner tube and to produce a permanent radial stress acting between said tubes.

8. The method of making a composite tubular device which comprises extruding an internally finned aluminum tube having a smooth outer surface, positioning an outer copper tube having a smooth inner surface over'said extruded aluminum tube, said aluminum tube afterextrusion havinga yield strength in excess of that of said outer copper tube prior to the draw operation, and thereafter drawing said tubes to produce a permanent reduction in the outside diameter of said inner tube and to produce a permanent radial stress acting between said tubes.

References Cited by the Examiner UNITED STATES PATENTS 813,918 2/1906 Schmitz. 1,441,459 1/ 1923 Small. 2,726,681 12/1955 Gaddis et a1. 13838 2,828,537 4/1958 Pischke et a1 29517 2,985,508 7/1959 Drake 138-38 3,009,484 11/1961 Dollens. 3,041,719 7/1962 Haseltine 29517l FOREIGN PATENTS 526,593 9/1940 Great Britain.

CHARLIE T. MOON, Primary Examiner. LEWIS r. LENNY, Examiner. 

1. THE METHOD OF MAKING A COMPOSITE TUBULAR DEVICE WHICH COMPRISES EXTRUDING A LINER TUBE HAVING A SMOOTH OUTER SURFACE AND A FINNED INNER SURFACE, ASSEMBLING OVER SAID LINER TUBE A SHROUD TUBE HAVING SMOOTH INNER AND OUTER SURFACES AND HAVING A YIELD STRENGTH SUBSTANTIALLY LESS THAN THE YIELD STRENGTH OF THE LINER TUBE, SINKING THE SHROUD TUBE ONTO THE LINER TUBE BY DRAWING THE ASSEMBLY THROUGH A DIE SIZED TO MATERIALLY STRESS THE LINER TUBE. 